1. Field of the Invention
The present invention relates generally to nuclear fuel rod quality control and, more particularly, is concerned with a method for nondestructive measurement of the heat affected zone of an identification code on a nuclear fuel rod tube.
2. Description of the Prior Art
Various techniques have been proposed in the past to identify specific nuclear fuel rods for different purposes. For instance, in U.S. Pat. No. 3,747,959, physical markings, such as indentations or notches, are used for object identification. In U.S. Pat. No. 4,636,846, fuel rods are stamped with indicia in the form of a character code prior to insertion into the reactor for the purpose of identifying each fuel rod upon removal after irradiation.
Bar code labels used for identifying retail products have also been used for identification of nuclear fuel rods. For instance, in U.S. Pat. No. 4,587,407, bar code labels are affixed to nuclear fuel rod claddings which contain information such as an individual identification number assigned to the particular fuel rod. The bar code is adapted to be optically scanned and read to determine its assigned number. The bar code labels are employed for establishing traceability of a fuel rod during its manufacture. The labels are manually applied at the start of the manufacturing process and manually removed at the end. Thus, there is no continued traceability of individual fuel rods after completion of manufacturing.
The use of bar codes would appear to have great promise for identification and traceability of individual fuel rods. However, their application on labels has shortcomings. The labels are costly to apply and remove. The labels will not stay on while the fuel rods are in use in a nuclear reactor.
More recently, as evidenced by the two patent applications cross-referenced above, a comprehensive approach to individual fuel rod identification has been devised, permitting traceability of individual fuel rods during all stages of both manufacture and use. This approach provides for engraving or etching a bar code directly on the circumferential surface portion of the cladding tube of the fuel rod by using the beam of a laser with the tube extending into a sealed chamber having a gaseous atmosphere of an argon-oxygen mixture. Laser marking of the Zircaloy fuel rod tube generates heat when the laser source comes in contact with the tube. The heat causes an oxidation of the Zircaloy and generates a readable bar code.
The heat also produces a change in the metallurgical microstructure of the tube below the bar code which change is called the "heat affected zone". Depending upon the depth of the zone, the tube could be weakened and cause problems later in the nuclear reactor. The depth of the change or heat affected zone is dependent on the amount of power generated by the laser which is used to etch the bar code on the tube.
Presently, a destructive test is used to measure the heat affected zone. One tube from a batch of a large number of tubes, such as 400-600 tubes, is destructively tested by cutting a cross-section through the tube and examining the cross-section metalligraphically to determined the extent of the zone. One shortcoming of this test is that it takes approximately four hours to take the measurements on the tube tested and to receive the results. If the heat affected zone of the tube is found to be of unacceptable depth, then all tubes of the batch are discarded. Another shortcoming is that, due to the inordinate time taken to complete the test, it is not feasible to test every tube. Thus, it is highly likely that many of the tubes discarded might actually be acceptable. Both the delays necessitated by performance of the destructive test and the likelihood that some acceptable tubes are rejected make the present testing technique extremely expensive.
Consequently, a need exists for an alternative approach to testing bar coded nuclear fuel rod tubes which will avoid the shortcoming of the prior testing technique.